1. Field of the Invention
The present invention relates to a method for recovering scraps of a multi-layer plastic sheet or film to be used as a wrapping material, for example More particularly, the present invention relates to a method for recovering scraps of a multi-layer sheet or film comprising different plastics for their effective reuse.
2. Background Information
With diversity of a manner of living, a wrapping material having a variety of functions has been desired in recent years. Thus a plastic material excellent in oxygen gas barrier properties and thermal resistance is desired to be developed in place of the conventional metallic can or glass bottle.
However, a single plastic material cannot meet the characteristics required. Thus a multi-layer sheet or film comprising different plastics or plastic films and metal films is increasingly used. For example, a multi-layer sheet or film comprising polyvinylidene chloride (hereinafter referred to as "PVDC"), an ethylene-vinyl alcohol copolymer (hereinafter referred to as "EVOH") or the like, which is excellent in oxygen barrier properties, and polypropylene (hereinafter referred to as "PP"), crystalline polyethylene terephthalate (hereinafter referred to as "crystalline PET") or the like, which is excellent in thermal resistance, is now widely used as a wrapping material for precooked food, particularly as a wrapping material for use in an electric range.
For production of such multi-layer sheets or films, a lamination method in which films are once produced and then laminated with an adhesive, for example, a coextrusion method in which a plurality of resins are extruded from different extruders and laminated by the use of a feed block or a multi-layer die, and a coextrusion lamination method which is a combination of the above lamination and coextrusion methods are well known.
The most important technical subject in use of the above multi-layer sheet or film is recovery of scraps produced at the time of production or fabrication thereof. In the case of a single plastic product, the reuse of the plastic can be usually achieved without a marked reduction in physical properties by carrying out its self-cycling. However, a serious problem is encountered in recovery of a multi-layer sheet or film as described above.
That is, in general, scraps produced at the time of production of a multi-layer sheet or film or at the time of its fabrication such as thermoforming are pulverized as such and recovered into a PP layer (a base material layer).
However, PVDC contained in the scraps or an adhesive resin is contained in the PP layer, causing a reduction in physical properties such as stiffness, impact strength and transparency.
Moreover, an oxygen gas barrier resin such as PVDC is generally high in heat sensitivity and particularly at a fabrication temperature of PP, is readily subject to thermal decomposition, causing problems such as discoloration and finally carbonization, and formation of black spots in the product.
When PVDC or an adhesive resin is contained in the PP layer (base material layer), for example, in the case of a multi-layer sheet or film having the construction: PP/adhesive resin/PVDC/adhesive resin/PP, PVDC or the adhesive resin is present in the surface layer, they cause problems such as corrosion of the inner surface of die and adhesion to the surface of a cooling metallic roll to be used in production of sheets and so on.
Therefore the following methods are employed in order to reuse in a stabilized manner scraps recovered (recovered starting material).
(1) Use of a special screw
Since PP, EVA, and PVDC having different extrusion characteristics are present together, a double fright type screw is used or a special plating is applied taking into a consideration a difference in melting initiation temperature.
(2) In order to prevent thermal deterioration of the recovered starting material, a thermal deterioration preventing additive is added.
(3) As PP to be used as the recovered layer, a low viscosity PP making an extrusion temperature not more than 210.degree. C. is used.
Even by these methods, however, the PP layer is decreased in mechanical properties such as stiffness and impact resistance because scraps contain low thermal resistant resins such as PVDC, EVA and an adhesive resin. Moreover, baking and discoloration inevitably occur. To make discoloration unmarked, it is necessary to add a large amount of an additive, e.g., a white pigment to the PP layer.
To overcome the problems due to the presence of PVDC or adhesive resin in the surface layer, it is necessary to provide another layer not containing scraps on the surface of the above layer. This makes the equipment very complicated.
Recovering scraps into the base material layer (PP layer) means introduction of expensive PVDC and adhesive resin into inexpensive PP layer. It is remarkably uneconomical.
The present inventors made extensive investigations to overcome the above prior art problems. As a result, it has been found that scraps can be reused without a reduction in physical properties and economically by separating them into at least two layers and recoverying each layer separated into the corresponding layer. Based on the findings, the present invention has been accomplished. That is, in accordance with the present invention, scraps are not pulverized for recovery and reuse thereof.